Helicopters are well known for the loud sound they make as their blades spin, but many people do not know what causes the noise. Researchers at the German Aerospace Center set out to explain the cause and provide a visual explanation for the noise.The researchers explained that most of the noise is generated by the helicopter’s main rotor. When the rotor spins, air pressure decreases above it and increases below it. This is what causes the helicopter to rise. Air flows around the blade to even out the pressure difference and creates a concentrated vortex. When the vortex meets the next blade, it produces sound vibrations.
The German researchers were able to capture that effect on camera. Air seems to shimmer on hot days because changes in air density refract light. They took advantage of that knowledge and were able to make the vortices created by the helicopter blades shimmer so that they could be photographed.
The researchers visualized the rotor blade vortices in 2013 by flying in front of a rocky background in the Alps and over fields and meadows. They were able to create 3D images of another helicopter’s rotor blade vortices when they took a helicopter to a limestone quarry and asked pilots to rock it only 30 feet above the ground in front of an exposed rock face. They set up 10 cameras at different angles to capture the effect.
The researchers plan to continue testing. They hope that that information they gain will help engineers make helicopters quieter and more comfortable for civilian and military pilots and passengers.